One of prior art hydraulic recovery devices equipped in hydraulic drive systems is described in JP-B-4-57881, for example. This hydraulic recovery device is equipped in a hydraulic drive system comprising a plurality of actuators operated by a hydraulic fluid supplied from a variable displacement type hydraulic pump, and a plurality of directional control valves disposed between the hydraulic pump and the plurality of actuators for controlling respective flows of the hydraulic fluid supplied to the associated actuators. The hydraulic recovery device comprises variadisposed in a first line communicating a reservoir port of at least one of the plurality of directional control valves and a reservoir, a third line communicating a portion of the first line upstream of the recovery control valve and a second line connected to a pump port of the directional control valve, and a check valve disposed in the third line for allowing-the hydraulic fluid to flow only in a direction from the first line toward the second line.
The recovery control valve comprises a spool formed with a variable throttle, a hydraulic driving sector to which the pressure in the second line is introduced as a condition variable related to the operation of a hydraulic cylinder for driving the spool in the valve opening direction, and a set spring for urging the spool in the valve closing direction. An opening area (an amount of restriction) of the variable throttle is set at the position where the pressure introduced to the hydraulic driving sector and the urging force of the set spring are balanced.
When the directional control valve is operated in a direction to extend a rod of the hydraulic cylinder as the associated actuator, the hydraulic fluid from the hydraulic pump is introduced to a bottom side hydraulic chamber of the hydraulic cylinder via the first line and the directional control valve. On the other hand, the hydraulic fluid delivered from a rod side hydraulic chamber with the operation of the hydraulic cylinder flows into the directional control valve and is then introduced to the reservoir via the first line and the variable throttle of the recovery control valve. At this time, while the load of the hydraulic cylinder is small and pressing force due to the pressure in the second line introduced to the hydraulic driving sector of the recovery control valve is smaller than the pressing force of the set spring, the variable throttle is held at its closed or throttled position and, therefore, a pressure corresponding to the amount of restriction is generated in the first line. At the time the pressure so generated in the first line exceeds the pressure in the second line, a part of the return fluid flowing out of the directional control valve to the first line is allowed to pass into the second line via the third line and the check valve for recovery, and is then supplied to the directional control valve after joining with the hydraulic fluid from the hydraulic pump. Thus, the flow rate of the hydraulic fluid supplied to the bottom side hydraulic chamber of the hydraulic cylinder is increased by an amount corresponding to the recovered flow rate introduced from the first line, thereby increasing a moving speed of the hydraulic cylinder accordingly.
On the other hand, when the load of the hydraulic cylinder is increased and the pressure in the bottom side hydraulic chamber is raised, the delivery pressure of the hydraulic pump is raised and so is the pressure in the second line introduced to the hydraulic driving sector of the recovery control valve. Therefore, the spool of the direction and the pressure in the first line is so reduced that the pressure in the second line becomes higher than the pressure in the first line. As a result, the check valve is held closed and driving force of the hydraulic cylinder is ensured against the large load.
In short, with the above-described prior art, when the load of the hydraulic cylinder is small, at least a part of the hydraulic fluid returned from the hydraulic cylinder to the reservoir is recovered and used for driving the hydraulic cylinder, whereby the moving speed of the hydraulic cylinder is increased and hence the working efficiency is improved. When the load of the hydraulic cylinder is increased, driving force of the hydraulic cylinder is also increased, enabling the load to be surely driven.
Another example of prior art hydraulic recovery devices is described in U.S. Pat. No. 5,168,705. This hydraulic recovery device is designed to employ a shift amount of the directional control valve as a condition variable related to the operation of the hydraulic cylinder, and to change an opening area of the variable throttle of the recovery control valve in link with the shift amount of the directional control valve. More specifically, the variable throttle of the recovery control valve is formed on the same spool as that of the directional control valve on which meter-in and meter-out variable throttles are formed. When the directional control valve is finely operated, the opening area of the variable throttle of the PG,7 recovery control valve is small as with the meter-in and meter-out variable throttles of the directional control valve, the pressure in the first line is so raised that a part of the hydraulic fluid can be recovered without causing a cavitation. When the shift amount of the directional control valve is increased, the opening area of the variable throttle of the recovery control valve becomes larger as with the meter-in and meter-out variable throttles of the directional control valve, the pressure in the first line is lowered and driving force of the hydraulic cylinder is ensured against the large load.